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Core (freeze) plugs need a sealant that keeps them seated under heat cycling and coolant pressure — and ideally one with a little give, since one job of these plugs is to pop out before a hard freeze cracks the block. A few thread-locker and RTV compounds have become the go-to choices in the engine-building community.
| # | Product | Price | Where to buy |
|---|---|---|---|
| 1 | Permatex Permanent Strength Red & Cup/Core Plug Sealant Retaining Compound | $10-$18 | Amazon |
| 2 | Loctite 680 Retaining Compound | $12-$20 | Amazon · eBay |
| 3 | Loctite 540 Thread Sealant | $15-$25 | Amazon |
| 4 | Permatex Form-A-Gasket No. 2 | $8-$14 | Amazon · eBay |
The picks, in detail
Permatex Permanent Strength Red & Cup/Core Plug Sealant Retaining Compound
A retaining compound formulated specifically for cup/core plug installation.
- High-strength anaerobic retaining formula
- Purpose-labeled for core/cup plugs
- Resists rust in the threaded/press-fit engagement area
- Formulated specifically for this job, not a generalist sealant
- Strong, reliable seal
- High strength can make plug removal difficult later if ever needed
Loctite 680 Retaining Compound
A green retaining compound with a long track record of sealing freeze plugs without leaks or pop-outs.
- High-strength anaerobic retaining compound
- Fills gaps in press-fit applications
- Widely used across engine-building forums for freeze plugs
- Proven long-term reliability reported by engine builders
- Good gap-filling for press-fit plugs
- Anaerobic cure needs metal-to-metal contact to set properly
Loctite 540 Thread Sealant
A thixotropic anaerobic sealant specifically suited to sealing core plugs.
- Thixotropic anaerobic formulation
- Designed for core plug sealing applications
- Prevents rust in the threaded engagement area
- Purpose-designed for exactly this application
- Corrosion protection built into the formula
- Less commonly stocked at general auto parts stores than Permatex products
Permatex Form-A-Gasket No. 2
A non-hardening sealant that engine builders have relied on for decades for freeze plugs.
- Non-hardening sealant formula
- Retains some flexibility after cure
- Long history of street-engine use
- Allows slight flex, helpful if a plug needs to pop under a hard freeze
- Inexpensive and easy to find
- Lower bonding strength than dedicated retaining compounds
Buying tips
- Clean the bore thoroughly before applying any sealant — contamination is the most common cause of a core plug leak, regardless of which sealant you use.
- If you want the plug to still pop under a genuine hard freeze (its intended safety function), a non-hardening sealant like Form-A-Gasket No. 2 is more appropriate than a high-strength retaining compound.
- Anaerobic retaining compounds (Loctite 680/540) generally give the most leak-proof seal for engines that will never see freezing conditions.
As an Amazon Associate and eBay Partner, toptenpick.com earns from qualifying purchases. Prices and availability are subject to change.